The present disclosure relates to the use of waste streams to generate energy. In particular, waste streams formed by the production of cellulosic ethanol are used herein. The disclosure also relates to fluidized-bed boilers, such as a bubbling fluidized-bed (BFB) or circulating fluidized-bed (CFB) boiler, for combusting such products and materials, and to related arts.
During combustion, the chemical energy in a fuel is converted to thermal heat inside the furnace of a boiler. The thermal heat is captured through heat-absorbing surfaces in the boiler to produce steam. Fuels used in the furnace include a wide range of solid, liquid, and gaseous substances. Combustion transforms the fuel into a large number of chemical compounds. In some applications, solid biomass waste byproducts are used as fuel for the fluidized-bed boiler.
Fluidized-bed boilers are one way to burn solid fuels. Generally speaking, a fluidized-bed boiler includes a bed formed from a stacked height of solid particles. A fluidization gas distribution grid, such as an open bottom system or a flat floor system, is located beneath the bed. An open-bottom system is characterized by widely spaced distribution ducts on which are mounted air bubble caps for distributing fluidizing gas (typically air) under pressure to fluidize the bed of solid particles. In a flat floor system, the distribution ducts form the floor of the boiler. At sufficient gas velocities, the solid particles exhibit liquid-like properties.
With reference to FIG. 1, an illustrative bubbling fluidized-bed (BFB) boiler 8 of a known design (available from The Babcock & Wilcox Company, Barberton, Ohio, USA) includes a bubbling bed 10 onto which fuel 12 is delivered via a feeder 14. The fluidized bed 10 suitably comprises solid particles such as, for example, sand. A gas-tight furnace flue (only the lower portion of which is shown in FIG. 1) includes gas-tight tube walls 16, 17 made up of tubes through which water flows to cool the walls. A fluidizing gas, such as air, is introduced into the bubbling bed 10 through ducts 18, and spaced-apart bubble caps 20 facilitate removal of large tramp material. In an underbed ash removal system 22, tramp material moves downward and cools before being removed through bottom hoppers 24 onto a suitable conveyor system or the like (not shown). Heat from combustion on the fluidized bed 10 heats water in the wall tubes 16, 17 which may drive a steam generator or other useful work. In some embodiments, water in the tube walls 16, 17 flows in a closed-loop recirculation path (usually including a make-up water line). The feeder 14 may pass through a non-water-cooled refractory furnace wall (e.g., a brick furnace wall) rather than through tube wall 16 as in the illustrative embodiment of FIG. 1, or through any other type of boiler wall. It is contemplated for the furnace wall through which the feeder 14 passes to include additional features such as thermal insulation material, an outer casing, or so forth.
Cellulosic ethanol is an advanced type of biofuel produced from wood, grasses, or the inedible parts of plants. This type of biofuel is produced from lignocellulose, a structural material that comprises much of the mass of plants. Lignocellulose is composed mainly of cellulose, hemicellulose and lignin. Corn stover, switchgrass, miscanthus grass, wood chips, agricultural residue, and even the byproducts of lawn and tree maintenance are some of the feedstock containing lignocellulose.
The production of cellulosic ethanol biofuel typically requires additional processing with specialty chemicals, enzymes, and microorganisms to break down the lignocellulose. As a result, the waste products of cellulosic ethanol production are significantly different from those of the traditional starch ethanol process, which primarily uses cereal grains (e.g. corn kernels) as the feedstock. There are typically two waste streams from the cellulosic ethanol biofuel process. One is a lignin filter cake (cake) with a typical moisture range of 35% to 60% and remainder solids. The second is a syrup with a typical moisture range of 30% to 50% and remainder solids.
Unlike the waste products of starch ethanol production, the waste products from the cellulosic ethanol biofuel process typically cannot be sold as either feed or fertilizer due to the chemistry of the waste products resulting from the specialty mixture of chemicals, enzymes, and microorganisms needed to produce this biofuel. It would be desirable to provide methods and devices that can be used to derive the useful energy contained in the cellulosic ethanol waste products.